This invention relates to a well-treating process for dissolving a siliceous material within a well borehole or a subterranean earth formation. More particularly it relates to increasing the amount of siliceous material which can be dissolved and retained in solution by certain high temperature formulations of self-generating mud acid fluids of the type described in the C. C. Templeton, E. H. Street, Jr., E. A. Richardson U.S. Pat. No. 3,828,854. That patent and/or the invention it describes is referred herein as the U.S. Pat. No. 3,828,854 or invention, and the disclosures of the patent are incorporated herein by cross-reference.
In accordance with the U.S. Pat. No. 3,828,854, siliceous materials in or around a well borehole are dissolved by contacting them with an aqueous liquid solution or dispersion of a water-soluble fluoride salt and a relatively slowly reactive acid-yielding material which subsequently converts the salt fluoride solution to a hydrofluoric acid solution which has a relatively high pH but is capable of dissolving siliceous material. The lower aliphatic alcohol esters of formic acid have been used in numerous wells and have proven to be highly satisfactory in reservoirs having temperatures between about 130.degree.and 200.degree. F. For use at higher temperatures a recommended class of acid-yielding material comprises similar alcohol esters of higher molecular weight water-soluble fatty acids. We have now discovered that, although such esters of the higher acids are generally suitable, they provide a level of net dissolved siliceous material (and thus total degree of reservoir permeability improvement) which tends to be significantly less than what is provided by the lower molecular weight acid lower temperature formulation.
The cause of the diminished effectiveness of the self-generating mud acid solutions containing such higher acid esters was not obvious. Prior to the discovery of the U.S. Pat. No. 3,828,854 invention, it had been generally believed that a mud acid solution would not be effective unless it contained enough strong acid (e.g., HCl) to provide a very low pH (of significantly less than 1). The clay-dissolving capabilities of the mud acid solutions of the 854 process demonstrated that the pH of a mud acid fluid could be initially near 7 and that such a fluid could dissolve significant amounts of clay or silica without the pH ever becoming less than about 2, and often remaining above about 4.
The E. A. Richardson U.S. Pat. No. 3,889,753 describes a discovery that, in a buffered mud acid fluid, the rate of silica dissolution can be retarded (more than the amount of the dissolution is retarded) by buffering the pH at a relatively high value. That patent describes "buffer-regulated" mud acid solutions comprising aqueous solutions of a weak acid, a weak acid salt and a fluoride salt, which solutions are capable of dissolving significant amounts of clay while maintaining pH's as high as about 4.9 to 5.2.
Our copending patent application, Ser. No. 754,143, filed Dec. 27, 1976, describes a discovery that, in a buffer-regulated mud acid solution of a weak acid, a weak acid salt, and a fluoride salt of the type described in the U.S. Pat. No. 3,889,753, the clay-dissolving capability can be increased by adding an aminopolyacetic acid salt chelating or complexing agent.